This invention relates to automotive electrical systems, especially systems used in large trucks and over-the road tractors. A particular aim of the invention is to provide large vehicles with the capability to supply other vehicles with electrical power to two selected voltages, e.g. 12 volts or 24 volts. Such power may be required when the other vehicle battery system is in a discharged (dead) condition. The present invention facilitates the process of jumping electrical power from a charged system to a discharged system, whether the discharged system is 12 volt or 24 volt.